By virtue of their very low basicity, anions of sulfonylimide type are increasingly used in the field of energy storage in the form of inorganic salts in batteries, or of organic salts in supercapacitors or in the field of ionic liquids. Since the battery market is in full expansion and reduction of battery manufacturing costs has become a major challenge, an inexpensive large-scale process for synthesizing anions of this type is necessary.
In the specific field of Li-ion batteries, the salt that is currently the most widely used is LiPF6, but this salt has many drawbacks such as limited thermal stability, sensitivity to hydrolysis and thus lower safety of the battery. Recently, novel salts bearing the group FSO2− have been studied and have demonstrated many advantages such as better ion conductivity and resistance to hydrolysis. One of these salts, LiFSI (LiN(FSO2)2) has shown very advantageous properties which make it a good candidate for replacing LiPF6.
Few processes for synthesizing LiFSI or the corresponding acid thereof have been described, but it is clearly seen that, in all these processes, the key step is the step of forming the S—F bond.
A first synthetic route described (Appel & Eisenbauer, Chem. Ber. 95, 246-8, 1962) consists in reacting fluorosulfonic acid (FSO3H) with urea. However, the corrosive and toxic nature of this compound does not allow industrialization of the process.
EP 2 415 709 describes a process based on this route, in which the products of reaction of fluorosulfonic acid with urea are dissolved in water and bis(fluorosulfonyl)imide is precipitated in the form of the salt with tetrabutylammonium. This synthetic route is not viable on a large scale since the overall yield is very low.
Another route consists in reacting difluorosulfoxide with ammonia: see WO 2010/113 835 in this regard. However, this method also forms numerous side products, which necessitates expensive purification steps.
Another route (Ruff & Lustig, Inorg. Synth. 1968, 11, 138-43) consists in synthesizing in a first stage a dichloro compound of formula (ClSO2)2NH and then in performing a chlorine/fluorine exchange with AsF3. However, this process is not industrializable due to the high price and the toxicity of AsF3.
WO 02/053 494 describes another route which consists of a Cl/F exchange on (ClSO2)2NH using a fluoride of a monovalent cation which may be alkaline or of onium type (NR4+), in an aprotic solvent. However, according to said document, the reaction is very slow.
Example 10 of WO 2007/068 822 describes the synthesis of bis(fluorosulfonyl)imide in anhydrous hydrofluoric acid (HF). Thus, the reaction is performed in an autoclave with 1 g of bis(chlorosulfonyl)imide and 4 g of anhydrous HF at various reaction temperatures and times. The document teaches that even at temperatures of 130° C., the reaction yield does not exceed 55%. In addition, it teaches that the presence of impurities makes separation difficult at the industrial scale. It concluded that the synthesis of bis(fluorosulfonyl)imide with HF is unsatisfactory, and thus that the use of a lithium fluoride is preferred during the chlorine/fluorine exchange step.
WO 2009/123 328 describes the manufacture of sulfonylimide compounds, via a reaction between amidosulfuric acid and thionyl chloride and then with chlorosulfonic acid, to form bis(chlorosulfonyl)imide, which is then subjected to a fluorination step. The fluorination is performed with a fluoro compound such as CuF2, ZnF2, SnF2, PbF2 or BiF3. However, these fluoro compounds are expensive, making the exploitation of the process at the industrial scale difficult.
There is thus still a need to produce imides containing a sulfonyl group (such as LiFSI), especially via a process that can be performed at the industrial scale.